As HVAC technicians, we are sometimes called to investigate problems with light commercial refrigeration. One of the more confusing components in a refrigeration system is a defrost control timers.
While there are many different ways to control defrost, in light commercial refrigeration a defrost control timer is most often used. In low temperature applications where the box is kept below 32°F, heat is needed to make sure the ice is completely removed from the evaporator during the defrost cycle.
The standard timer has a timer wheel that represents the 24 hours in a day, it also has 12 threaded holes and 4 matching screws (defrost initiation pins). See below.
The defrost initiation pins are screwed into the threaded holes that correspond with times the freezer will be defrosted. The defrost is usually initiated 4 times a day, and should be set for times when the freezer is not in use or immediately following normal stocking times.
The timer also has an inner wheel that is settable for up to 120 minutes of defrost. This timer is not the primary means of terminating defrost, but acts as a back-up should the primary defrost termination device fail. This back-up defrost termination is usually set for 20 minutes.
*Please refer to your specific timer’s wiring diagram as different manufacturers may use different terminals than used in this example.
Most timers have a copper jumper between two of the terminals, in this instance between 2 and 3. This jumper can be discarded but, in most cases it is not. The timer motor itself is powered by terminals 3-2 and X.
There are 2 normally closed (NC) contacts and 1 normally open (NO) contact. The NC contacts are connect to the compressor and the evaporator fans and the NO contacts are connected to the defrost heater.
Finally there is a defrost termination solenoid connected between terminals 1 and X. This solenoid is the primary means of defrost termination and, when energized, physically kicks the timer out of defrost. The defrost termination solenoid is controlled by a defrost termination fan delay stat often called a Klixon named after a manufacturer of this stat.
It is also important to understand the defrost termination fan delay stat. This device has a common lead (usually 120 Volts) and a close on temperature rise lead and a close on temperature fall lead. Let’s break this into its 2 sections:
First we have the close on temperature rise lead. During the defrost cycle, the defrost timer de-energizes the fan(s) and turns on the defrost heaters. As the frost begins to melt and the temperature of the evaporator coil rises to the close on rise set point, 120 Volts is applied to the defrost termination solenoid, ending the defrost cycle.
As the defrost cycle ends 120 Volts is applied to the fan(s) but the close on temperature fall lead is wired in series with the fans. The fan(s) will not come on until the temperature of the coil falls to the close on fall set point. This ensures that the heat from the defrost heaters does not blow into the freezer.
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by Ron Walker